The present invention generally concerns power conversion circuits.
A number of different electronic devices require very fast modulation of their supply voltage. One such type of electronic devices is radio frequency (rf) linear power amplifiers. Such amplifiers are widely used in modern wireless communication devices and infrastructure. In complex modulation schemes commonly used in wireless communications like QPSK, CDMA, WCDMA, the amplitude of the envelope of the rf signal varies significantly. At every instance when the envelope of the rf signal is substantially lower than the maximum allowed by the supply voltage, the efficiency of the power amplifiers is severely reduced. In other words, a significant portion of the supply energy is expensed only for maintaining the power amplifier's operating point (bias) without creating useful signal. There are a number of adverse effects caused by this phenomenon, including (i) the need to oversize the expensive rf components in the amplifier system, (ii) increased cooling requirements, (iii) increased size and weight of equipment, and (iv) increased consumption of electrical energy. If, on the other hand, the supply voltage is changed in accordance with the envelope of the rf signal, the operating point of the power amplifiers can be kept at or near optimum at all times. As a result, efficiency can be maintained at a high level, regardless of the instantaneous amplitude of the envelope of the rf signal.
However, while rf power amplifiers ordinarily require very fast modulation of their supply voltage for improved efficiency, most available electronic energy sources are designed to maintain a constant, well-regulated output voltage and are required to vary their output voltage only at relatively slow rates. For example, the CDMA baseband frequency is 1.25 MHz and the WCDMA baseband frequency is 5 MHz. This results in an rf signal envelope having the most energy in the band 0-1.25 MHz and 0-5 MHz respectively. Multichannel amplifiers, on the other hand, experience envelope variations due to the interactions between different carrier frequencies. In such a situation, the rf signal envelope experiences amplitude variation with frequency components reaching the difference in carrier frequency of extreme channels (two channels with the greatest difference of the carrier frequency). The envelope frequency in this case can be on the order of hundreds of kHz to tens of MHz. If the bandwidth of the power supply is insufficient, distortion results and additional noise in the communication channels emerge, which results in an increased error rate in the communication channel. The present modulation rate goals are two to three orders of magnitude greater than what can be achieved by simply modulating a pwm signal of traditional dc-dc converters. This makes traditional pwm dc-dc converters unsuitable as power supplies for devices, such as rf power amplifiers, that require ultrafast modulation of their supply voltage.
FIG. 14 illustrates a prior art linear regulator 1400. The regulator 1400 includes a preamplifier stage 1402 and an output stage 1404. The preamplifier stage 1402 includes a preamplifier 1406, which may include a set of discrete components, or may be realized as a fully integrated circuit. An input signal is provided to an input terminal 1401 of the preamplifier 1406. The output of the preamplifier 1406 is provided to a pair of discrete power transistors 1408, 1410 arranged in a push-pull configuration. The proper bias (dc operating point) of transistors 1408, 1410 is provided by a pair of regulated voltage generating circuits 1414, 1416. The voltage generated by the circuits 1414, 1416 is selected to cancel the non-active input voltage region of the transistors 1408, 1410 at low input voltage levels. The transistors 1408, 1410 are of opposite types. Transistor 1408 is an n-type power Field Effect Transistor (FET) or an npn-type power bipolar transistor, while transistor 1410 is a p-type power FET or a pnp-type power bipolar transistor. An output terminal 1403 is provided at the junction between transistor 1408 and the transistor 1410. A feedback line 1412 provides a feedback signal to the preamplifier 1406, causing it to amplify the difference between the input and output voltages. When the output voltage of the preamplifier 1406 is below the input voltage, the output of the preamplifier 1406 goes up and the transistor 1408 is biased on, sourcing current to any load present at the output terminal 1403 and bringing the output voltage to the desired level. The transistor 1410 is in cut-off. When the output voltage of the preamplifier 1406 is above the input voltage, the output of the preamplifier 1406 goes down and the transistor 1410 is biased on, sinking current from any load present at the output terminal 1403 and thus bringing the output voltage to the desired level. The transistor 1408 is in cut-off.